1. Field of the Invention
The present invention relates generally to video encoders and decoder systems, and in particular, to a method of splicing compressed video pictures from selected encoder sources to achieve seamless switching.
2. Description of the Related Art
Television studios today typically process noncompressed, analog video. Video signals in the analog domain in a television studio are all synchronized, that is, the starting, running and ending times of each video picture is the same. Since each picture starts at the same time, switching from one analog encoder to another analog encoder requires the simple procedure of monitoring the end of a video picture in the first encoder since the beginning and end of each picture of each video signal is synchronized with the beginning and end of each picture in the second encoder. Since there is synchronization, switching out of one picture at the end thereof automatically places the video at the beginning of a picture in the other selected video signal. Therefore, switching, for example, from one video source (showing for example, a television network program) to another video source (showing for example, a local affiliate's commercial) when using analog (and therefore synchronized) signals, is relatively simple and well known.
Switching in the digital domain however, introduces additional obstacles that must be overcome. Generally, a video source may be transmitting either digitally and uncompressed, or digitally and compressed, video signals. In the first situation where the digital signal is uncompressed, switching is still relatively simple since the signals again can be easily synchronized as is well known in the art. In this way, switching from one video encoder source to a second video encoder source when dealing with digitally uncompressed signals is similar to the methods found when switching in the analog domain.
However, with the introduction of HDTV and other high resolution display devices, there is an increasing desire to transmit and broadcast compressed digital video signals to effectively operate over the necessary wider bandwidth. One known standard for transmitting in the compressed digital domain is MPEG. MPEG represents a compressed video picture by a number of bits. However, the difficulty is that the particular number of bits for each picture can vary. That is, the number of bits representing one video picture has little correlation to the number of bits representing the next video picture. Therefore, editing and switching between video sources, as illustrated above, of digitally compressed video signals becomes very difficult because it is necessary to establish a smooth and accurate splicing time between the selected video sources.
The MPEG standard does not set forth a solution to this problem. Although MPEG is a variable rate encoding standard, the output of many MPEG encoders must be transmitted over a channel (for example, over a television network) using a different rate that depends on the channel and system characteristics.
Therefore in such a system, the encoder formats data in a variable manner while the transmission system is capable of only transmitting the video data bits at a different bit rate. Therefore, without some sort of "buffer" to manage the flow of video data, problems will arise. The MPEG standard along with the state of the art encoder/decoder systems recognizes this problem. One suggested technique to solve the problem is the introduction of an encoder buffer disposed between the encoding system (but which may still be part of the encoder) and the channel. The buffer is constructed to smooth the variable rate of data generated by the encoder to the bit rate that can be transmitted over the channel and to the decoder. This buffer has been coined an encoder buffer.
However, the introduction of the aforementioned encoder buffer introduces a still further significant problem. That is, there must be buffer management to manage the data flow within the encoder buffer to make sure it does not overflow with video data. That is, if data is entering the encoder buffer too quickly as compared to the rate at which it is outputted to the channel, the encoder buffer will at some point overflow with data. This will cause a loss of video data which may not subsequently be recapturable. On the other hand, if the data entering the encoder buffer is entering at a very low rate, i.e. slower than the rate at which it is being outputted to the channel, there may be an underflow of data at the encoder.
The decoder has a decoder buffer that gets data from the channel and has data taken out periodically by the decoder. Decoder buffer underflow and overflow causes unacceptable artifacts. The encoder buffer must therefore be managed in such a way as to ensure that the decoder buffer does not underflow or overflow.
Another suggested method of achieving seamless splicing of compressed digital video is to decompress the video signal, switch the video from the first selected encoder to the second selected encoder in a known manner, and then again reencode the video. However, this method is extremely expensive, requiring additional circuitry, because it requires decompression of the video, switching thereof, and thereafter, recompression. Moreover, because switching in this manner requires the encoding and decoding of a video signal twice, there may be an appreciable reduction in the video signal quality.
Therefore, an improved method of switching between two or more selected encoders (which are transmitting digitally compressed video) to achieve at least essentially seamless splicing and overcome the aforementioned deficiencies and disadvantages is desired.